Analog and digital communication have long used frequency/wavelength multiplexing as one means of achieving greater bandwidth. Through multiplexing, discrete signals defined by distinct wavelengths are transmitted across the same medium. Each discrete signal is typically assigned to carry specific information. Signal attenuation within a fiber network, however, is oftentimes frequency or wavelength dependent. Accordingly, the rate of attenuation, commonly measured in dB/km can vary among different wavelengths within a fixed optical spectrum. Consequently, the wavelength distinguished by the highest rate of attenuation will typically govern fundamental network parameters such as the maximum distance between repeaters. One result of differing rates of attenuation, therefore, is that different wavelengths transmitted at a same power will be at different power levels upon reaching a repeater or other processing station. In addition, the routing and switching of signals within a metropolitan network has the capacity to combine signals of disparate power levels. Moreover, there is unevenness in the multiplexing and demultiplexing components, unequal gain over different wavelengths in erbium doped fiber amplifiers (EDFAs), unequal laser launch power for the different channels, etc. All of these features exacerbate the uneven power levels of different wavelengths during the transmission, re-transmission, routing and processing of an optical signal.
FIG. 1 illustrates a spectrum made up of many discrete wavelengths, from a first wavelength λ1 up to an nth wavelength λn, which form component signals within a collective wavelength multiplexed signal within an optical medium. The Y-axis represents signal power, and the X-axis represents a spectrum of wavelengths. The lower signal threshold 124 is the lowest signal power level to which a signal may attenuate and remain reliably processable according to system requirements. The “saturation threshold” 120 is the maximum allowable signal power of the network for any one wavelength. Between these two levels, a reference power level 122 is illustrated throughout FIGS. 1, 2 and 4 for comparative purposes only. For illustrative purposes, it is assumed that all of the component wavelengths or frequencies depicted in FIG. 1 began at equal signal strength, and have attenuated to the levels seen in FIG. 1 during launch, transmission, routing or other processing within a fiber optical network. As seen in FIG. 1, the signals can be at different strengths. The third wavelength λ3 is seen to be quite robust, remaining above the reference level 122. Whereas, the fourth wavelength λ4 is seen to have attenuated to a signal strength substantially below the reference level 122.
FIGS. 2 and 3 show the signals of FIG. 1 after each component wavelength has been uniformly amplified. Because the third wavelength λ3 was the strongest signal prior to amplification, it remains the strongest signal after amplification. Plotting uniformly amplified signals, the relationship in signal strength is therefore unchanged from the pre-amplification relationship of FIG. 1, provided all of the component signals remain below the saturation threshold. FIG. 2 shows all component signals within the upper limit of the network parameters, with the strongest signal, the third wavelength, λ3, at the upper limit. As noted however, the other discrete wavelengths fall far below the upper threshold. Because it was earlier determined that the fourth wavelength λ4 was subject to the greatest attenuation during transmission, future transmission subsequent FIG. 2 is limited by the fourth wavelength λ4, which is both the weakest signal, and subject to the greatest attenuation. Failure to amplify the fourth wavelength λ4 to the maximum allowable signal strength 120 will result in attenuation of λ4 to the lower threshold 124 in a substantially shorter transmission distance than if it had begun at the upper threshold 120. Alternatively, FIG. 3 shows the fourth λ4, which is the weakest component wavelength in the figure, amplified to the upper threshold 120. The problem with this approach, however, becomes clear when an examination is made of the other component signals in FIG. 3. By amplifying the weakest signal up to the upper threshold 120 of the network, in a uniform amplification process, all other signals, λ1, λ2, λ3, λn are amplified above the upper threshold 120 of the optical network.
To optimize network performance therefore, a first step in the processing of a wavelength multiplexed signal is channel equalization of component signals λ1, . . . , λn. FIG. 4 illustrates component signals in a wavelength multiplexed signal which have been both equalized, and amplified to the upper threshold 120 of the network parameters. Unless the weakest component signals λ1, . . . , λn is below the allowable threshold for maintaining an acceptable signal to noise ratio, the first step of the equalization process is to reduce the signal strength of each component wavelengths component signals λ1, . . . , λn to the level of the lowest power of any of the signals present. Alternatively, the component signals may be reduced to a common pre-determined power level. The second step in the equalization process is to uniformly amplify the equalized component signals λ1, . . . , λn to a predetermined power level, preferably the maximum recommended power level 120 (FIG. 4) of a network. By this process of equalization and amplification, all wavelengths within a signal can be equally amplified to the maximum power allowable on a fiber network, thereby maximizing the signal to noise ratio and minimizing the bit error rate. Equalization is used in this context to indicate that all signals have been attenuated to an equal level. Equalization is also used in a more general sense to refer to any desired attenuation level, which can vary from wavelength to wavelength.
U.S. application Ser. No. 10/051,972 filed on Jan. 15, 2002, and entitled “METHOD AND APPARATUS FOR DYNAMIC EQUALIZATION IN WAVELENGTH DIVISION MULTIPLEXING” teaches a channel equalizer in which a grating light valve™ light modulator array is used for dynamic signal equalization of component wavelengths within a wavelength division multiplexed (WDM) optical fiber network. The WDM signal is de-multiplexed into its component wavelengths. The component wavelengths are appropriately directed such that each component wavelength impinges a grating light valve™ light modulator of the grating light valve™ light modulator array. The grating light valve™ light modulator array equalizes each component wavelength. The equalized component wavelengths are re-multiplexed and output as an equalized WDM output signal via an output fiber.
The Ser. No. 10/051,972 application teaches a separate detecting system that monitors the power levels of each equalized component wavelength. A coupler, or fiber tap, is positioned to divert a portion of the equalized WDM output signal to an optical performance monitor in order to measure the power levels associated with each equalized component wavelength. The measured power levels are provided as feedback to a controller, which in turn adjusts the grating light valve™ light modulator array until equalization is reached. According to this approach, a separate de-multiplexer would be required to de-multiplex the diverted signal to measure the strength of the component wavelengths making up the diverted portion of the equalized WDM output signal. This approach suffers from the obvious need for redundant de-multiplexor/multiplexor and imaging lenses, and is therefore inefficient and costly.
The Ser. No. 10/051,972 application further describes an alternative embodiment in which separate couplers and light sensors are integral to the channel equalizer of the Ser. No. 10/051,972 application. This approach requires that each component wavelength be directed by a waveguide, and that a coupler and a sensor are attached to each waveguide.
There exists therefore a need for a method and apparatus for efficiently and economically measuring the power levels on the individual equalized wavelength channels. There is also a need for a method and apparatus for measuring the power levels on the individual equalized wavelength channels without coupling to each individual fiber transmitting a de-multiplexed wavelength channel. There further exists a need for a method and apparatus for measuring the power levels on the individual equalized wavelength channels without coupling to a single fiber transmitting multiplexed equalized wavelength channels that require de-multiplexing prior to measuring.